Conventionally, in MPEG-2 (Moving Picture Experts Group 2) and other image coding schemes, ideal allocation of code amounts has been a significant challenge for maintaining subjective image quality.
For example, code amount allocation ideal for a still image refers to the state that the still image is encoded with uniform distortion (fixed quantizer scale). With highly-distorted code amounts, distortions can be gathered into high frequency components or complicated areas for improved subjective image quality.
For example, Patent Document 1 discloses a method of coding an image quality signal, in which code amount control of so-called feedforward system is employed to enable controls adaptive to local properties of the image quality signal, thereby allowing a decoded image of improved image quality. In the feedforward system, the amounts of code to be generated from an equal length unit are calculated in advance for quantizer steps based on a plurality of quantizer scales, and an appropriate quantizer scale is determined beforehand within the extent that the generated code amount will not exceed a target code amount.
Meanwhile, in proposed MPEG-2 test model TM5 and the like, code amount control is effected by means of feedback control using the relationship between the remaining level of a virtual buffer, quantizer steps used in previous encoding, and generated code amounts.    Patent Document 1: International Publication WO96/28937.
According to the foregoing still-image based conventional art, however, it is necessary to calculate code amounts and make predictions with different quantizer steps plural times in order to find out an average quantizer step that is close to the target code amount and provides uniformity across the entire screen. This increases the cost pertaining to the circuits for calculation.